In many processes, such as in a chemical process, it is desirable to have a specific chemical moiety, group or fragment available so that it can provide a beneficial and useful effect. These are called useful groups (UG). However, in some cases, the useful group cannot be directly present in the system or process but must be incorporated as a precursor group in a precursor compound. In such a compound the active part (useful group) that causes the useful effect is chemically blocked or is otherwise unavailable and then is converted or de-blocked into its active form at some point after the process has begun. Often, it is desirable that this conversion or de-blocking occurs as a function of cleavage of a specific chemical bond within the precursor compound. In such cases, there is an unstable intermediate group (sometimes referred to as a blocking group) that chemically connects the UG to that part of the molecule where the bond is broken during the process. After the bond is broken, the intermediate subsequently decomposes to release the UG.
For example, processes are known where reducing bacteria are added to a redox carrier that contains a blocked and shifted dye. The bacteria reduce the redox carrier to a form where the dye is released and provides optical density in a region different from its blocked form. Thus, an increase in optical density is a measure of the number or strength of reducing bacteria. In this example, the UG is the released dye and the redox carrier serves as a kind of chemical switch that causes the conversion of the inactive UG (the blocked dye) into its useful form as a function of a broken chemical bond (due to the bacteria). There are many different kinds of blocking groups that are known to release UGs under specific conditions or process.
As a particular example, it is well known in the photographic art to use compounds that upon reaction with oxidized developer (Dox) release various types of photographically useful groups (PUGs) in an imagewise fashion. There are many known ways to accomplish the release of a PUG upon reaction with Dox.
For example, the PUG can be chemically attached directly to the site of reaction with Dox such that the bond connecting the PUG to the rest of the compound is broken and the free PUG fragment released. Alternatively, the PUG can be attached to the rest of the compound indirectly through the use of an unstable intermediate group. Upon reaction with Dox, the unstable intermediate group still bearing the PUG is released and the free PUG fragment is then produced only after a subsequent decomposition step. It should be noted that reaction with Dox to release a PUG could be either via coupling (in which the oxidized developer becomes chemically bound to part of the compound) or via a redox reaction (in which the oxidized developer is reduced and the compound oxidized).
In the photographic art, these unstable intermediate groups that connect a PUG to a site of reaction with Dox are commonly referred to as timing or switching groups. These unstable groups have also been occasionally described as linking groups, but the term “linking group” is more correctly applied to intermediate groups that are stable and remain part of the PUG after reaction with Dox.
The function of these timing groups is widely varied and depends heavily on the requirements of the photographic use, the nature of the PUG and concerns over other issues such as rate of reaction with Dox, long-term keeping and synthesis. For example, it is often desirable for the timing group to decompose slowly on the development timescale so the -(timing group)-PUG complex can diffuse away from the site of reaction with Dox and thus release PUG in a remote location. In other uses, it is desirable for the timing group to decompose quickly such that the PUG is released almost instantaneously. In a similar manner, it may be desirable to increase the water solubility of a timing group in order to maximize diffusion in some uses, but decrease the water solubility and increase the molecular weight by adding an oil soluble ballasting group in order to restrict diffusion for other uses.
When the PUG is a fragment that inhibits silver development, compounds that release the inhibitor directly upon reaction with Dox are commonly referred to as DIRs (development inhibitor releasers). Compounds with an unstable timing group between an inhibitor fragment and the Dox reaction site are generally referred to as DIARs (development inhibitor assisted releasers).
Many different types of timing groups are known. For example, compounds that release a PUG via an intramolecular nucleophilic displacement have been described in U.S. Pat. No. 4,248,962, U.S. Pat. No. 4,857,440 and U.S. Pat. No. 4,847,185. Compounds that release a PUG via an electron transfer along a conjugated system have been described in U.S. Pat. No. 4,409,323, U.S. Pat. No. 4,859,578, U.S. Pat. No. 5,576,167, U.S. Pat. No. 4,421,845, U.S. Pat. No. 4,477,563 and U.S. Pat. No. 5,326,680. U.S. Pat. No. 4,933,989 describes a timing group that undergoes an intermediate redox reaction to release a PUG.
Despite a large number of attempts to provide UG releasing compounds with desirable performance, there still remains a need for materials with improved properties. In a particular embodiment, the problem remains to provide a silver halide photographic element having the desired tone scale with improved image structure and excellent color reproduction.